In digital communications, channel estimation is needed to compensate for disturbances ubiquitous on a radio channel. Channel estimation and compensation are needed to achieve high data rates. Pilot tones or one or more sub-carriers may be transmitted when transmitting modulated information signals. Pilot tones may be used for channel equalization or transmission power control. Known symbols may also be inserted in a sequence of time-sequential data symbols.
OFDM is used in a variety of communications systems such as digital television, digital audio broadcasting, or digital radio, wireless LAN and various wireless communication systems, e.g. IEEE 802.11 (a and g), IEEE 802.16 (d and e) and CDMA2000.
A research proposal titled, Optimal Channel Estimation for Capacity Maximization in OFDM Systems, by Wu et al, Mar. 26, 2003, University of Texas Austin, USA, describes channel estimation techniques in designing OFDM systems. A theoretical tool for analyzing and comparing two different estimation techniques is proposed. The intent is to compare a frequency domain pilot tone method with time domain training sequence methods. From the optimization model, various parameters of the channel estimation can be tuned to maximize system capacity.
In the research proposal, the two channel estimation techniques for OFDM channel estimation are referred to as frequency domain pilot interpolation, FDPI, and time domain training sequence, TDTS. With FDPI, pilot tones are transmitted. With TDTS, a known pseudo-noise (PN) sequence is inserted at the beginning of a symbol in the time domain. The cross correlation between the received PN sequence and the known PN sequence can be used to estimate the channel response at the receiver. The proposal intends to develop analytical models and simulation tools to optimize pilot arrangement patterns or training sequence parameters to achieve maximum capacity.
FIG. 1 illustrates pilot arrangements discussed in the research proposal. There are two basic types of pilot tone arrangements: a comb-type pilot arrangement <<Comb-type>> and a block-type pilot arrangement <<Block-type>>. For the comb-type pilot arrangement, example pilot tones <<p1>>, <<p2>> are transmitted. Channel gain/attenuation at the pilot frequencies <<p1>>, <<p2>> is measured, and the channel gain/attenuation at intermediate frequencies is obtained by interpolation. For the block-type pilot arrangement, all sub-carriers are used as pilot tones at regular intervals. In the figure, two time-instances or time-intervals are illustrated <<t1>>, <<t2>>.
An article titled, Pilot Tone Selection for Channel Estimation in a Mobile OFDM System, by Negi et al., IEEE Trans. on Consumer Electronics, Vol. 44, No. 3, August 1998, addresses the issue of selecting pilot tones for channel estimation. Negi et al. conclude that the best sets of tones to be used are those with equal spacing. Based on a first order Markov channel model, they propose that it is more efficient to use a few pilot tones in all symbols than to use all tones as pilot tones in some symbols.
Another article, Channel Estimation Techniques Based on Pilot Arrangement in OFDM Systems, by Coleri et al., IEEE Trans. on Broadcasting, vol. 48, No. 3, pp. 223-229, September 2002, investigates channel estimation techniques for OFDM systems based on pilot arrangement. Channel estimation based on a comb-type pilot arrangement is studied through different algorithms for both estimating a channel at pilot frequencies and interpolating the channel. In addition, channel estimation based on block-type pilot arrangement is performed by sending pilots at every sub-channel and using this estimation for a specific number of following symbols. A decision feedback equalizer for all sub-channels combined with periodic block-type pilot channel estimation is also implemented.
OFDM channel estimation by singular value decomposition, IEEE Trans. on Communications, by Edfors et al., vol. 46, pp. 931-939, July 1998, Analysis of DFT-based channel estimation for OFDM, Wireless Personal Communications, by Edfors et al., vol. 12, No. 1, pp. 55-70, January 2000, and Analysis of low-complexity windowed DFT-based MMSE channel estimator for OFDM systems, by Yang et al. IEEE Trans. on Communications, vol. 49, pp. 1977-1987, November 2001, describe Least Square, LS, Minimum Mean Square Error, MMSE, or Discrete Fourier Transform, DFT, based estimators.
TELECOMMUNICATIONS INDUSTRY ASSOCIATION TIA-864, Recommended Minimum Performance Standards for cdma2000® High Rate Packet Data Access Network Equipment, February 2002 describes a forward pilot channel, an unmodulated, direct-sequence spread spectrum signal transmitted every half slot in bursts of 96 chips centered at the middle of the half slot. The Pilot Channel allows an access terminal to acquire the timing of the Forward Channel, provides a phase reference for coherent demodulation, and provides means for signal strength comparisons between sectors for determining when to handoff. It also describes a forward MAC channel, a forward channel used for medium access control. The Forward MAC Channel consists of Reverse Power Control Channels, DRCLock Channel and Reverse Activity Channel. Within each slot, the Pilot, MAC, and Traffic or Control Channels shall be time-division multiplexed. All time-division multiplexed channels shall be transmitted at equal power. A Forward Traffic Channel, a forward channel used to transport user and signaling traffic from an access network to an access terminal, is also described.
1×EV-DO Platinum Multicast is suggested as a backwards-compatible evolution of 1×EV-DO. The existing EV-DO forward link design uses CDMA to transmit data packets to a single user (uni-cast), or simultaneously to multiple users (multicast), during different time slots (called time division multiplexing, or TDM). Each packet is provided the full forward link power from one cell sector during its time slot. EV-DO Platinum Multicast further improves performance by reserving the same TDM time slot at all cells in a region and then transmitting one or more common packets within the reserved slot to all users in the region. Mobile devices receive the same packet from multiple cells and then softly combining the energy to improve reception. To simplify the soft combination of the common packets transmitted simultaneously from all cell sectors in multi-cast mode, an orthogonal frequency division multiplex (OFDM) waveform is used for transmission during the common TDM time slot.
Parameter Estimation: Principles and Problems, by H. W. Sorenson, New York, N.Y., Marcel Dekker, 1980, derives an MMSE estimator for a channel matrix H based on observation vector Xp of pilot sub-carriers.